1. Technical Field of the Invention
The present invention relates generally to a valve timing control system for internal combustion engines which is so designed as to control the timing of opening and closing of intake or exhaust valves to improve drivability of the engine in the event of failure of one of intake or exhaust valve timing control mechanisms.
2. Background Art
In recent years, automotive internal combustion engines equipped with variable valve timing control mechanisms working to vary the timing of opening and closing of intake and exhaust valves have increased in order to improve engine output, fuel economy, and exhaust emissions.
Japanese Patent First Publication No. 10-121999 discloses an automotive valve timing control device for internal combustion engines equipped with two variable valve timing control mechanisms one for each of two cylinder groups. If one of the variable valve timing control mechanisms has failed, the valve timing control device works to control the other variable valve timing control device to decrease the amount of overlap between the opening and closing of inlet and exhaust valves of a corresponding one of the cylinder groups.
Specifically, the valve timing control device works to decrease the amount of overlap of the intake and exhaust valves of one of the cylinder groups operating normally to secure the stability of burning of the engine. It is, however, impossible to change the amount of overlap of the intake and exhaust valves of the cylinder group which is malfunctioning, which may cause the amount of EGR (exhaust gas recirculation) to be increased in some engine operating range, thus resulting in instability of engine combustion. Specifically, one of the cylinder groups which is in service is stable in combustion, while the other cylinder group which is out of service is still instable in combustion, thus resulting in a variation in output torque of the engine.